Aug 4, 2015 - 8:53 AM
in reply to agassizmevl
If I understand your plans correctly you need a train ticket from Geneva to Mürren via Montreux and Zweisimmen. It can be purchased in Geneva or online. Additionally you need a (boat) ticket from Montreux to the Château de Chillon and back. You can buy that in Montreux.
The Swiss Half Fare Card that Slowpoke mentioned might be worth looking into. It allows you to buy those tickets with a 50% discount and also comes in handy in Mürren as I assume you'll be using cable cars and trains there too.
Tickets are good for the day so you can make stops wherever you like. The ticket from Montreux to the castle might be time limited, but you'll buy it on the spot so that's no problem.